Reassured it was an original, she texted her managers and set up a studio session. In a matter of weeks, the voice memo on her phone had transformed into an extravagant, string-laden ballad.

After that, an entire album came together with surprising speed.

"My lifestyle's really busy now, with all my children and everything, so I'd get to the studio at nine in the morning and... just put my mind to it.

"I only had three sessions and I wrote six songs. Two songs every session and I was home for school time!"

Fans expecting Stefani to knock out some ska-punk covers of Bing Crosby; or an electro-hip-hop take on Jingle Bells will be sorely disappointed, though.

The star, who grew up listening to Emmylou Harris's Christmas record, Light Of The Stable, and the Charlie Brown Christmas Album, stays affectionately loyal to the old standards, sprinkling her arrangements with chirpy stabs of brass and twinkling, doo-wop backing vocals.

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The singer found fame in the ska/new wave band No Doubt, whose biggest hit was Don't Speak

Even her version of Wham's Last Christmas adds a 60s sheen to George Michael's heartbroken classic.

"That was kind of the vibe we wanted: That wall of sound, Ronettes-style, throwback," she says.

"It's really rare to be able to do a record like this because it's so expensive to get all the live musicians in.

"It's so old-fashioned [but] it was pretty magical to be part of that."

'Sexiest man alive'

The album, You Make It Feel Like Christmas, is a refreshing palate-cleanser after Stefani's last solo release, 2016's This Is What The Truth Feels Like.

That record came in the wake of her divorce from Gavin Rossdale and a subsequent, unexpected, romance with Blake Shelton - her co-host on the US version of The Voice.

The results were suitably confused, swerving between the icy barbs of Me Without You and the cartoonish rebound pop of Make Me Like You; without settling on a consistent tone.

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Stefani and Shelton met on The Voice US and have been dating since 2015

Stefani's personal life is off-limits, the BBC is told in advance of this interview - but she happily chats away about Shelton, after pausing to check whether we've heard of him.

"Exactly!" Stefani laughs, with a self-deprecating flick of her hair. "I don't know if you heard about him? The sexiest man alive? That's my boyfriend, by the way."

In fact, it was going to be hard to avoid discussing Shelton, since he co-wrote the title track on You Make It Feel Like Christmas, a cute little love song that rides the rhythm of The Supremes' You Can't Hurry Love.

The track came out of the blue, says Stefani, as Shelton has largely given up writing his own songs.